The present invention relates, in general, to position transmitters and, in particular, to a new and useful position transmitter which can be incorporated in a pneumatic-pneumatic or electro-pneumatic converter, and which readily provides for calibration between a zero position and a maximum position of a device to be actuated by the converter.
Several methods have been utilized in the past for determining the position of an actuated device such as a piston or mechanical drive unit. These methods sensed the position of an element of the actuator device and transmitted this position back to the system controller for controlling the device. One of the more basic methods utilized a variable resistive element, such as a slide wire or potentiometer. This variable resistive element would have a constant voltage applied to it and its slider or wiper would be mechanically linked to the actuator device. By moving the slider, a variable voltage was generated between the wiper and one side of the resistive element. This variable voltage corresponded to the position of the device. The variable voltage was transmitted to the system controller.
A more sophisticated method utilized a so-called LVDT (linear variable differential transformer) or RVDT (rotary variable differential transformer) as the position sensing element. Interface circuitry converted the variable signal of the LVDT or RVDT to a corresponding current signal which was normally a 4-20 mA signal. This signal was transmitted to the system controller.
A potentiometer was also used as a position sensing element with interface circuitry providing the variable current signal. These transmitters were usually not integrally part of the electro-pneumatic or pneumatic-pneumatic positioner used to operate the controller which, in turn, operated the device to be controlled.